1. Field of the Technology
The present disclosure is directed to thermal mechanical treatment of high strength precipitation hardening martensitic stainless steels. In particular, a thermal mechanical treatment is disclosed that includes hot working and direct aging.
2. Description of the Background of the Technology
Significant efforts have been made to formulate certain stainless steel alloys, such as martensitic precipitation hardening (PH) stainless steel alloys, that exhibit superior properties for use in high performance articles. The potential for excellent strength-to-weight ratios, toughness, corrosion resistance, and stress corrosion cracking (SCC) resistance of articles formed from these alloys make them particularly well suited for use as aerospace structural components such as, for example, flap tracks, actuators, engine mounts, and landing gear hardware. These properties, along with various manufacturing considerations, are strongly influenced by alloy composition, structure, heat treatment, and level of process control in the alloy systems. To obtain the properties necessary for high performance applications, careful and strict control of the alloying components and their concentrations and ratios is generally required. Even slight variations in the identities, concentrations, or ratios of alloying components can significantly affect the properties and performance of these stainless steel alloys.
For example, early forms of martensitic stainless steel alloys employed copper as the major hardening element. Alloys 17-4PH and 15-5PH, for example, were developed by coupling copper addition with high chromium levels and moderate levels of nickel. These early forms of steel alloys are recognized as having good corrosion and SCC resistance, but have been found to have relatively low yield strength levels (YS<180 ksi). Because of the relatively inferior strength properties exhibited by martensitic stainless steel alloys including copper additions, copper has not been favored as a major strengthening element in high strength stainless steel alloys.
Other martensitic stainless steel alloys have been developed that employ various levels of aluminum to enhance strength. These alloys can exhibit yield strength greater than 200 ksi in the H950 condition (i.e., aged at an aging temperature of 950° F.), along with good ductility and toughness. However, the strength of this type of martensitic steel is still relatively low and may be insufficient for many high strength applications. Other martensitic stainless steel alloys have been developed that employ both aluminum and copper as strengthening elements. These alloys exhibit much higher strengths (YS≧235 ksi), but fail to achieve acceptable levels of fracture toughness (KIC<65 ksi·in1/2).
Other approaches to forming martensitic stainless steel alloys involve the addition of titanium as the major strengthening element along with various levels of copper as the secondary strengthener, and providing a suitable nickel-chromium equivalence. Alloys formulated by these approaches provide relatively high strength (YS>240 ksi) and good corrosion resistance, but exhibit low toughness (Charpy V-notch impact toughness (CVN)<10 ft/lb and KIC<65 ksi·in1/2).
More recent developments include the addition of relatively high levels of titanium (1.5%-1.8% by weight) and nickel, which achieves high toughness, but at the possible expense of corrosion resistance and SCC resistance due to nickel/chromium imbalance. These alloying systems also involve a costly and time consuming cryogenic treatment step after solution heat treatment in order to achieve their high performance properties.
Still other high strength martensitic steel alloys employ a combination of aluminum and titanium as strengthening agents. These alloys can be divided into two groups: 1) alloys that employ relatively low levels of aluminum and titanium and exhibit relatively high toughness; and 2) alloys that employ relatively higher levels of aluminum and titanium and exhibit relatively high strength. However, it has been found that the steel alloys of this type that exhibit high strength generally exhibit low toughness, with Charpy impact energies of only a few foot-pounds and facture toughness less than 60 ksi·in1/2 at room temperature.
More recently developed alloys, such a Custom 465® alloy and MLX17 alloy, exhibit both high strength and high toughness, include relatively high levels of aluminum and titanium hardening elements, and also include increased levels of the toughening element nickel. The concentration of nickel in these alloys, however, is increased to a level at which conventional solution-age treatments cannot be used, and expensive post-solution treatment cryogenic processing is required to obtain the increased mechanical properties.
Other approaches to formulating high strength steel alloys involve additions of one or more of silicon, beryllium, and molybdenum as hardening elements to form steel alloys with very high strength, but low toughness. Because of their low toughness properties, these steel alloys typically are unsuitable for high performance structural applications.
A relatively new stainless steel that achieves high toughness and high strength without the requirement for cryogenic treatment is disclosed in U.S. Pat. App. Pub. No. 2005/0126662 (“the '662 publication), which is hereby incorporated by reference herein in its entirety. The '662 publication discloses a precipitation hardening martensitic stainless steel alloy that exhibits excellent mechanical properties and high corrosion/stress corrosion cracking (SCC) resistance. The '662 publication's stainless steel includes controlled amounts of aluminum, copper, and titanium as hardening elements, together with carefully adjusted matrix chemistry, especially relating to levels of chromium, molybdenum, nickel, and, optionally, tungsten, boron, and carbon. This stainless steel can be processed by a conventional solution-age treatment without using expensive and time-consuming cryogenic treatments, as are required with some of the newly developed precipitation hardening martensitic stainless steels. While the corrosion/SCC resistance properties of the precipitation hardening martensitic stainless steel disclosed in the '662 publication are equal to or better than those of the newer, cryogenic-treated stainless steels, the ultimate tensile strength of the alloy disclosed in the patent publication is slightly lower at lower aging temperature conditions.
Accordingly, there continues to be a need for precipitation hardening martensitic stainless steels having advantageous mechanical properties that render the alloys suitable for certain high performance applications.